CR2 and COVID-19: Those that express CD27 (CD27+CD21−) are referred to as activated memory cells (acMBC), while those that express low CD21 and CD27 are called atypical MBCs (atMBC); both are expanded in chronic infections, autoimmune diseases, and shortly after acute infection and vaccination including COVID-19 vaccines35, 38, 39, 40, 41, 42, 43, 44.